How to rename Animation Node's Invoke Subprogram;s Generator Output?
Yes, it is possible to rename the Outputs of the Invoke Subprogram node and there are two ways to do that:

Select the Output Node:


Open the N-panel or properties panel --> Node --> Advanced Node Settings, there you can rename the output (blue box)
Or, press the U-key (which is a shortcut for the Output Node Settings), then a menu will appear in which you can rename the outputs (green box)